P2P Package Manager

Has anyone ever heard of a package manager that 'scans' other machines within its subnet or within specified subnets for updates before using the official repositories? There would either have to be some service advertisement protocol lime MDNS or each machine would literally have to scan for a designated port number listening for these requests on all machines. Once they locate each other, the idea would be ome machine downloads 500MB of updates from the repo, and from there on, every other machine (with the same distro and arch) just pulls from the faster local machine, rather than using up inet bandwidth.

Any suggestions (Other than a dedicated local repository mirror)?